Walker was given a full-time Formula One seat to drive the Lotus 72 in the 1972 season, as number two driver to Emerson Fittipaldi. As the season went on, however, both Walker and the team became increasingly disenchanted. After Lotus discovered Walker had tested a Formula Two car for another team, he was dropped from the team for the Italian GP and the Canadian GP, where he was replaced by Reine Wisell. Walker was back for the US GP, but retired.