He was a lawyer, and in 1855 became president of the Göta hovrätt, or lord justice for the appellate court of Götaland. From 7 April 1858 to 3 June 1870 he was Prime Minister for Justice and again from 11 May 1875 to 20 March 1876. From 1867 to 1878 he was the member for Stockholm in the first chamber in the New Riksdag, and introduced and passed many useful reforms.
The reforms introduced a bi-cameral elected Riksdag replacing the existing cumbersome and less democratic Riksdag of the Estates, a hangover from the later Medieval Times.
This measure was accepted by the Riksdag in December 1865, and received the royal sanction on 22 June 1866. Foreign some time after this De Geer enjoyed considerable popularity.
He retired from the ministry in 1870, but took office again, as Prime Minister of Justice in 1875. In 1876 he became the first Prime Minister of Sweden following a reform where the previous offices of Prime Minister for Justice (which he held at the time) and Prime Minister for Foreign Affairs where changed into Minister for Justice and a Minister for Foreign Affairs.
He served until April 1880, when the failure of his repeated efforts to settle the armaments question again induced him to resign.
From 1881 to 1888 he was Chancellor for the Universities of Uppsala and Lund. De Geer was a member in the Swedish Academy from 1862, on Seat 17.
He was an advocate of free trade and economic liberalism and some argue laid the foundations for the strong economic growth in Sweden from 1870 to 1970.
Royal Swedish Academy of Sciences. Swedish Academy]
As a member of the nobility he took part in the Swedish Riksdag of the Estates from 1851 onwards. In 1862, he was also elected a member of the Royal Swedish Academy of Sciences.