Nigeria forward Lookman continued his positive start to life in Italy seconds after half-time in Bergamo with his third goal in as many games which gave fans celebrating Atalanta's 115th birthday more reason to party.
His strike at the Gewiss Stadium moved Atalanta a point ahead of Napoli, who can reclaim the summit with a home win over Bologna on Sunday evening.
Lookman, 24, clipped a neat finish into the top corner after collecting Brandon Soppy's pass to net for the fourth time this term and maintain an unbeaten start to the season for Gian Piero Gasperini's side.
"It was Atalanta's birthday today so it was an important day for us and for us to get the three points, we're very happy with that," Lookman told 'DAZN'. "I'm glad to be getting these opportunities and hopefully I can keep on putting them in the back of the net."
Atalanta claimed the three points also thanks to Mario Pasalic, who levelled in the final minute of the first half with a well-taken first goal of the season, shortly after Giorgos Kyriakopoulos had opened the scoring for Sassuolo with an unstoppable volley.
Alessio Dionisi's Sassuolo, who sit ninth, gave a good account of themselves and were unlucky to not equalise just after the hour mark.
Substitute Domenico Berardi almost marked his first appearance since late August with a stunning goal in the 63rd minute when he let rip with a powerful inswinging effort which smashed out off the underside of the bar.
However the Italy forward succumbed to a thigh injury again with 11 minutes remaining, leaving the field with his shirt covering his face after completing less than a quarter of the match, and the hosts had no problems seeing out their seventh win of the campaign.