In May 2006 he was chosen as the SPD candidate for the Hanover mayoral election on 10 September 2006 against the CDU politician Dirk Topeffer and Ingrid Wagemann of Alliance '90/The Greens.

Due to legal restrictions, Weil was automatically removed from the office of mayor when he became Minister President of Lower Saxony on 19 February 2013.

Just weeks before the state election, opinion polls indicated that Weil, with the help of the Greens, would easily defeat incumbent Minister-President David McAllister.

At the time, his victory constituted the twelfth consecutive setback in a state vote for Chancellor Angela Merkel’s CDU party and therefore was widely interpreted as indicative for the national elections later that year.

[10] As Lower Saxony has a 20 percent stake in Volkswagen (VW), Weil has been an ex-officio member of the company's supervisory board since February 2013.

This had endangered Weil's position because it hypothetically would have enabled the CDU to elect their leader Bernd Althusmann as Minister President by a motion of no confidence.

In 2024, in response to the threat of mass layoffs at Volkswagen, Weil wanted to restore government subsidies for the purchase of electric cars.

[18] In 1987, Weil married public health expert Rosemarie Kerkow-Weil (born 1954), the former president of Leibniz University Hannover who teaches at the HAWK Hochschule Hildesheim/Holzminden/Göttingen.