He studied law in Geneva, Paris and Heidelberg.
In 1874 he was hired as a deputy prosecutor.
In 1878 he married Anne Louise Eggly, with whom he had four children.
In 1881 Lachenal was elected by the Great Council to the Senate.
From 1880 to 1892 he was a member of the Parliament of the canton of Geneva. From 1885 to 1892 he was an assistant judge at the Geneva cantonal Court. Lachenal was inducted into the Masonic Lodge "Fidélité et Prudence". He was also chairman of the Military Court .
Twice he was President of the National Council. In December 1892 he was elected to the Federal Council, where he served until 1899. In 1896, he was President. After his resignation from the state government, he remained for nearly two decades in politics.