| 30/11/1817 | Born in Garding (Schleswig) as eldest of four children of the pastor Jens Mommsen |
| 1821 | Move to Oldesloe (Holstein), private lessons with his father |
| 1/7/1832 | Birth of the publisher's daughter Marie Reimer, his future wife |
| 1834–1838 | Pupil at the grammar school Christianeum in Altona |
| 1838–1843 | Law studies in Kiel; member of the fraternity Albertina |
| 1843 | PhD thesis: De collegiis et sodaliciis Romanorum |
| 1843 | Liederbuch dreier Freunde (with Tycho Mommsen and Theodor Storm; Schwers'sche Verlagsbuchhandlung, Kiel) |
| 1844 | Die römischen Tribus in administrativer Beziehung (J. F. Hammerich, Altona) |
| 1844–1847 | Danish travel scholarship: study tour through Italy |
| 1845 | Oskische Studien (Verlag der Nicolaischen Buchhandlung, Berlin) |
| 1848 | Works as a journalist in Rendsburg, agitation against Denmark |
| 1848–1851 | Associate Professor of Roman Law at the University of Leipzig (dismissed for involvement in the May Uprising of 1848/49) |
| 1850 | Die unteritalischen Dialekte (G. Wigand, Leipzig) |
| 1850 | Über das römische Münzwesen (Weidmannsche Buchhandlung, Leipzig) |
| 1851 | Private scholarship, financed by the publishers Georg Wigand, Karl August Reimer and Salomon Hirzel |
| 1852 | Full Professorship in Roman Law at the University of Zurich |
| 1852 | Inschriften des Königreichs Neapel |
| since 1853 | Foundation and management of the large-scale project Corpus Inscriptionum Latinarum |
| 1854–1856 | Römische Geschichte. Volumes 1–3 (Weidmannsche Buchhandlung, Berlin) | |
| 10/9/1854 | Marriage to Maria Auguste (called Marie) Reimer | |
| 1854 | Full Professor of Roman Law at the University of Breslau (today: Wroclaw) | |
| 1854 | Inscriptiones confoederationis helveticae latinae (Meyer und Zeller, Zürich) | |
| 28/6/1855 | Birth of daughter Marie († 15/9/1936), who married Ulrich von Wilamowitz-Moellendorff on 20/9/1878 | |
| 1857 | Professorship at the Prussian Academy of Sciences | |
| 1857 | Corresponding member of the Göttingen Academy of Sciences | |
| 1857 | Corresponding member of the Verein für siebenbürgische Landeskunde (Society for Transylvanian Regional Studies) | |
| 23/3/1857 | Birth of son Wolfgang († 2/2/1930) | |
| July–December 1857 | Inscription tour to Austria, Hungary, Slovakia, Romania and Istria | |
| 1858 | Die römische Chronologie bis auf Caesar (Weidmannsche Buchhandlung, Berlin) | |
| 1859 | Member of the Central Directorate of the German Archaeological Institute | |
| 26/6/1859 | Birth of daughter Lisbet († 28/8/1910) | |
| 1860 | Geschichte des römischen Münzwesens (Weidmannsche Buchhandlung, Berlin) | |
| September–October 1860 | Research trip to Munich, Brussels, The Hague, Antwerp, Leyden, Utrecht and Amsterdam | |
| 1861–1888 | Full Professorship for Roman History at the University of Berlin | |
| January–July 1862 | Inscription tour to Italy and Dalmatia | |
| 19/4/1861 | Birth of son Karl († 28/7/1922) | |
| 19/7/1862 | Birth of son Kurt († 12/4/1869) | |
| 1863–1866 | Member of the Prussian House of Representatives | |
| April / May 1863 | Research stay in Paris, work on the translation of Geschichte des römischen Münzwesens; audience with Napoléon III. | Correspondence between Theodor and Marie Mommsen |
| 8/7/1863 | Birth of son Ernst († 14/3/1930) | |
| 1864 / 1879 | Römische Forschungen. 2 volumes (Weidmannsche Buchhandlung, Berlin) | Approx. 1170 letters |
| 8/7/1864 | Birth of daughter Käthe († 10/11/1880) | Period: 1854–1902 |
| 12/7/1865 | Birth of son Oswald († 11/11/1907) | |
| 10/6/1866 | Birth of daughter Hildegard († 29/7/1951) | |
| September/October 1866 | Inscription tour in Austria and Istria | |
| March–October 1867 | Edition trip to Italy and Paris | |
| 1867 | Foreign corresponding member of the Hungarian Academy of Sciences | |
| 7/1/1869 | Birth of daughter Adelheid († 14/10/1953) | |
| March / April 1869 | Inscription tour in Italy, prematurely broken off due to the death of son Kurt (12/4/1869) | |
| 19/2/1870 | Birth of daughter Luise († 11/12/1957) | |
| March–May 1871 | Inscription journey to Northern Italy | |
| 10/5/1871 | Birth of son Konrad († 4/11/1946) | |
| 1871 – 1888 | Römisches Staatsrecht. 3 volumes (Verlag von S. Hirzel, Leipzig) | |
| 1872 | Corpus Iuris Civilis (with P. Krüger; Weidmannsche Buchhandlung, Berlin) | |
| 21/7/1872 | Birth of daughter Anna († 11/1/1953) | |
| 1873–1879 | Member of the Prussian House of Representatives | |
| February–June 1873 | Edition trip to Rome | |
| 13/7/1873 | Birth of son Hans († 23/1/1941) | |
| 1873 | Analecta Liviana (with G. Studemund; Verlag von S. Hirzel, Leipzig) | |
| 1873 | Honorary Member of the Academy of Kazan | |
| September 1874 | Inscription tour to Austria, Croatia and Hungary | |
| 21/10/1874 | Birth of son Max († 22/12/1874) | |
| 1874/75 | Rector of the University of Berlin | |
| 1874–1895 | Secretary of the Philosophical-Historical Class of the Prussian Academy of Sciences | |
| 1875–1884 | Member of the Central Directorate of the Monumenta Germaniae Historica, head of the department Auctores Antiquissimi | |
| 1876 | Admission to the Accademia dei Lincei | |
| April–July 1876 | Edition trip to Rome and Naples | |
| 4/8/1876 | Birth of son Otto († 23/8/1877) | |
| September–November 1877 | Edition trip to Rome, Sardinia and Sicily, among others | |
| March–May 1878 | Edition trip to Rome and Sicily, among other places, accompanied by daughter Marie | |
| September–November 1879 | Edition tour to Switzerland, Italy and Austria | |
| 1880 | Fire in his study, loss of important manuscripts | |
| 1881–1883 | Member of the Reichstag | |
| 1882 | Iordanis Romana et Getica (Weidmannsche Buchhandlung, Berlin) | |
| March / May 1882 | Research trip to Rome and Florence, among others | |
| 1883 | Res Gestae Divi Augusti ex Monumentis Ancyrano et Apolloniensi (Weidmannsche Buchhandlung, Berlin) | |
| April–June 1883 | Research trip to Rome and Florence, among others | |
| 1885 | Römische Geschichte. vol. 5 (Weidmannsche Buchhandlung, Berlin) | |
| March–June 1885 | Inscription tour to Vienna, Rome and Florence, among others | |
| August–October 1885 | Research stay in Oxford, London, Cheltenham, Cambridge and Paris | |
| April–July 1888 | Edition trip to Rome and Florence (in May Mommsen's wife visited Rome for the first time) | |
| February–April 1889 | Edition trip to Paris, London, Brussels | |
| December 1890 | Limes Conference in Heidelberg | |
| 1892 | Reichslimeskommission | |
| May–June 1892 | Edition trip to Munich, Vienna and Pest as well as stay in Heidelberg | |
| 1892 – 1898 | Chronica Minora. 3 volumes (Weidmannsche Buchhandlung, Berlin) | |
| 1893 | Abriss des römischen Staatsrechts (Verlag von Duncker & Humblot, Leipzig) | |
| October / November 1893 | Edition trip to Rome accompanied by wife and daughter Hilde | |
| 1894 | Cassiodori Senatoris Variae (Weidmannsche Buchhandlung, Berlin) | |
| 1895 | Associé étranger of the Académie des Inscriptions et Belles-Lettres | |
| 1896 | Appointed honorary citizen of Rome | |
| January–May 1896 | Mommsen's last trip to Italy (accompanied by his daughter Luise) to Rome, Florence and Bologna, among other places. | |
| 1898 | Liber Pontificalis (Weidmannsche Buchhandlung, Berlin) | |
| 1899 | Römisches Strafrecht (Verlag von Duncker & Humblot, Leipzig) | |
| June / July 1899 | Research trip to Paris, Turin and Ivrea (accompanied by daughter Adelheid) | |
| 1899–1900 | Honorary chairman of the Goethe-Bund for the Preservation of Artistic Freedom in Germany | |
| 1901 | "Spahn affair“ | |
| April 1901 | (Last) research trip to Paris (accompanied by son Hans) | |
| 1902 | Wife Marie suffers stroke († 6/3/1907) | |
| 1902 | Nobel Prize for Literature | |
| 1903 | Rufin, Historia ecclesiastica (J. C. Hinrichs, Leipzig) |
| 1.11.1903 | Dies in Charlottenburg |
| 1905 | Theodosiani libri XVI cum constitutionibus Sirmondianis et leges novellae ad Theodosianum pertinentes (with P. Meyer; Weidmannsche Buchhandlung, Berlin) |
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