France international Lemar struck his first goal since April 2019 to break the deadlock in the 56th minute, appearing at the back post to guide in a cross from Kieran Trippier and complete a sweeping counter-attack.
Llorente sealed the victory in the 72nd minute, latching on to a header from Trippier and showing remarkable pace and power to race past a defender and confidently dispatch his shot into the bottom corner.
Winger Lemar, who has been somewhat lacklustre since joining in from Monaco June 2018, put in an impressive showing against Valladolid after netting his first goal of the season.
"Lemar's performance makes me really happy, he's been working for a long time wanting to show the player he is," said Atletico boss Diego Simeone.
"He hasn't had the consistency he would have wanted, to have the place he should have in the team. Now let's hope these performances give him confidence and security and composure. We need all the players to get to the objectives the club has."
A seventh straight league victory took Atletico top of the table on 26 points after 10 games, two ahead of second-placed Real Sociedad who have played 11. Their next league match is away to city rivals and reigning La Liga champions Real Madrid.