The French centre-back has signed for Benfica until 2029, ending his time at the Rojiblancos two years after joining from Barcelona.
At 31, Lenglet arrived at the Metropolitano with a prominent role in Diego Simeone's plans.
That importance gradually faded, though, after a run of errors involving cards and penalties overshadowed what had otherwise been strong form in possession and in building play from the back.
His first season told a different story. Lenglet featured in 36 matches on loan from Barcelona, scoring three goals and providing two assists across 3,045 minutes, and was an important figure in the side.
Once that loan spell ended, he severed his ties with Barcelona and signed for Atletico as a free agent.
The second campaign proved far quieter. He made 24 appearances and provided one assist across 1,694 minutes, operating for much of the season as the squad's fifth-choice centre-back.
Lenglet now heads to Portugal for a new challenge, the fourth country of his career after spells in France, Spain and England.
His path to the top began at Sevilla, where strong form prompted Barcelona to make a significant investment for his signature back in 2018.
He enjoyed three productive seasons in Catalonia before losing his place in the side.
From there, he moved through loan spells at Tottenham and Aston Villa, with a high salary at Barcelona complicating any permanent exit, before that year on loan at Atletico Madrid reignited his career and led to his free transfer.
Lenglet will now look to establish himself in Portugal, joining a Benfica side that can offer him the regular football and central defensive role that eluded him during his second campaign as a Rojiblanco.
At 31, and with four different leagues now on his CV, the Frenchman will be hoping Lisbon provides the fresh start his career currently needs.
Atletico, meanwhile, move on from a defender whose Metropolitano chapter began with genuine promise before fading into a peripheral role by the end.