He was involved in the development projects of the Swiss Helvetas [de] in Paraguay and Haiti between 1987 and 1989[6] and was a member of the board at Pro Natura between 2000 and 2010,[6] when he resigned to assume office as a member of the National Council of Switzerland.[7] Between 2010 until 2015 he was a member of the board at ecos [de].[6]
Jans joined the SP in 1998 and became a member of the Grand Council of Basel Stadt in 2001.[6] In the Grand Council, he was a member of the Commission of Economics and Taxes.[6] He remained a member of the Grand Council after he joined the National Council of Switzerland in 2010[7] succeeding Ruedi Rechsteiner[8] and only resigned in 2011.[9] He was re-elected to the National Council in the federal elections in 2011,[10] 2015[11] and 2019.[12] In the National Council, he was also member of the Commission of Economics and Taxes.[13] As it became clear that Anita Fetz would not stand for another term in the Council of States for Basel-Stadt,[14] he put himself forward as a candidate, but later withdrew to enable a female candidate for the SP.[15] He was again a candidate for the National Council and Eva Herzog for the Council of States. Both candidacies were successful in the October 2019 federal elections.[16]
He was elected vice-president of the SP in 2015, succeeding Jaqueline Fehr,[17] but resigned in 2015 when the party abolished the offices of the president and vice-president and instituted a co-presidency of Mattea Meyer and Cédric Wermuth.[18]
He was elected into the Executive Council of Basel-Stadt on the 25 October 2020[19] and as its president in November 2020.[20]Sarah Wyss of the SP succeeded him, after he resigned as a member of the National Council in December 2020.[21]
During the 2023 Swiss federal election he became an official candidate for Federal Council to succeed Alain Berset for the Social Democratic Party.[22] The election was held on 13 December 2023 and he was the official nominee together with fellow politician Jon Pult. Three ballot rounds were required to determine the successor. During the second ballot round, Jans was ahead of his counter parties receiving 112 votes, opposing candidate Daniel Jositsch received 70 votes (presumably from the Swiss People's Party and The Liberals).[23] He was ultimately elected to Federal Council during the third round of ballot.[24][25][26] Jans assumed office on 1 January 2024.
On 11 June 2004, Jans married American-born Tracy Renee Glass (b. 1972), a biostatistician originally from Miami, Florida, whom he met while hiking in Hawaii. They had a long distance relationship for two years before she relocated to Basel to take a position at Novartis, a large pharmaceutical company. She currently works as team lead at Swiss Tropical and Public Health Institute.[27][28] They have two daughters.[29][30] His wife and his children are Swiss-American dual citizens.[31][32][33]