Stoneman was selected by the Expos in the expansion draft and quickly established himself as their ace starter. He tossed a 7-0 no-hitter at Philadelphia on April 17, 1969 in his fifth major league start. He finished the year 11-19, 4.39, the only pitcher on the staff with more than seven wins, but led the NL in walks. He also established a record as the easiest pitcher to strike out, fanning 55 times in 73 at-bats that year, 75 percent of the time; he hit just .086 for his career. Stoneman dropped to 7-15 in 1970, but had his best season in 1971, going 17-16 (his only winning record) with a 3.14 ERA, tying for the NL lead in starts, and leading again in walks.
Stoneman threw his second no-hitter on October 2, 1972, a 7-0 victory over the Mets in which he walked eight; it was the first major league no-hitter pitched outside the United States. He was 12-14 with a 2.98 ERA that year, but declined thereafter. When he was sold to the Angels at the start of 1974, he was the Expos’ team leader in wins (51), shutouts (15), and strikeouts (851); he still holds the team record for strikeouts in a season (251 in 1971). He went on to work his way up through the Expos’ front office and became general manager in 1987.