In June 2019, Lammens stated his intention of running for Chief of Government (mayor) of Buenos Aires in that year's election as part of a broad front in opposition to the Republican Proposal-led government, composed of progressive and centre-left parties and leaders, saying that "Kirchnerism is not enough in the City". He confirmed his pre-candidacy (ahead of the simultaneous and mandatory open primaries) on 19 June 2019, as part of the Frente de Todos, the peronist coalition that endorsed the presidential candidacy of Alberto Fernández.
At the election on 27 October 2019, Lammens received a 687.026 votes – or 35% –, losing in the first round against Rodríguez Larreta's 55.9%. Despite the wide loss margin, Lammens's share of the votes represents the best electoral result for an opposition party in Buenos Aires City since Mauricio Macri's victory in 2007.
On 10 December 2019, Lammens was appointed by incoming president Alberto Fernández as minister of the newly restored Ministry of Tourism and Sports, succeeding Gustavo Santos and Diógenes de Urquiza, respectively the secretaries of Tourism and of Sports in Mauricio Macri's cabinet. Sports-wise, Lammens stated his intention of focusing on neighbourhood clubs.