Though still eligible for junior hockey, Mosienko left the team as they did not cover the cost of transportation for him within Winnipeg, later noting that as he was from a poor family he needed that money to help him out.

Mosienko and his linemates Clint Smith and Doug Bentley combined to score 219 points, at that time an NHL record.

[14] In 1944–45, Mosienko finished sixth in league scoring with 54 points and did not record a single penalty minute throughout the course of the season.

[9] He was awarded the Lady Byng Memorial Trophy as the NHL's most gentlemanly player and named a second-team All-Star.

[16] The following season, Chicago coach Johnny Gottselig paired Mosienko with the Bentley brothers: Max and Doug.

[6] Max Bentley led the league in scoring in 1945–46, and despite missing time with a knee injury,[17] Mosienko finished sixth.

[9] He played in the first National Hockey League All-Star Game prior to the start of the 1947–48 season, during which he suffered a broken leg and was initially feared lost for the year.

[18] He ultimately missed the first two months of play, while the Pony Line was broken up for good shortly after his injury when Max Bentley was traded to the Toronto Maple Leafs.

[9] Mosienko remained a key contributor to the Black Hawks offence, appearing in five All-Star Games during his career.

He scored three goals in a 21-second span of the third period against New York's Lorne Anderson to set a new record for the fastest hat-trick by one player.

[9] Due to his small size, Mosienko was a frequent target of physical abuse from larger opponents,[20] and suffered numerous injuries as a result.

Feeling the Black Hawks lowballed him in regards to salary, Mosienko retired a second and final time from the NHL in 1955.

[4] Additionally, Mosienko is honoured by the Manitoba Hockey Hall of Fame and in 2000 was named to its provincial All-Century first All-Star team.

[34][35] The younger Mosienko remembers his grandfather as a humble man who would join him on the family's backyard rink when he was learning to skate as a child.