Seidl’s move to McLaren was announced last month, but the team did not reveal his start date at that point.
It has now specified that he will commence work at McLaren from May 1, just after the Azerbaijan Grand Prix.
McLaren chief executive Zak Brown said the team was “delighted” with when Seidl would be able to complete his move.
“This enables us to continue the momentum of our recovery plan and we’re looking forward to welcoming him to McLaren,” Brown added.
Seidl has previous F1 experience from Williams and BMW, and led the latter’s DTM return before his switch to Porsche.
He is one of two high-profile signings McLaren has so far made as it revamps its technical line-up in a bid to turn its recent poor form around.
Former Toro Rosso technical director James Key is also moving to McLaren.
Though Key is expected to arrive during the 2019 season, the timing of his switch has been complicated by Toro Rosso’s parent company Red Bull’s reluctance to release him early..