During the 1976 Major League Baseball expansion draft, Stein was selected by the Seattle Mariners, who took him with their third pick in the draft. Stein stated he was "glad" that the Mariners drafted him, because he did not like playing at Comiskey Park, the home of his previous team, the Chicago White Sox. He was profiled by the Associated Press during spring training in 1977, and was interviewed about his new team and his unique versatility in the field. Stein mentioned to the reporter that although he had played a wide variety of positions in the past, he hoped he would get a chance to be the Mariners starting third baseman. In a win against the Boston Red Sox on May 3, Stein hit two home runs in the same game. In June, Stein commented on how he liked starting every day at third base for the Mariners. The Mariners manager, Darrell Johnson, Praised Stein for playing "good ball" with Seattle. On July 8, in a game against the Minnesota Twins, Stein had another two-home run performance, his second of the season. In early-September, Stein got hit in the shoulder by a baseball, which was later revealed to have caused a hairline fracture. With the Mariners that year, he batted .259 with 53 runs scored, 144 hits, 26 doubles, five triples, 13 home runs, and 67 RBIs in 151 games played. Defensively, the vast majority of his games (147) were played at third base, but he also played limited time at shortstop. He led the American League in putouts by a third baseman with 146. Stein was also fifth in the league in defensive games at third base.