Pentti was born in Karijoki, Finland in 1925 and came to Thunder Bay in Canada as a six-year old. He got all his hockey training over in North America.
Before he came to the NHL he played for the Port Arthur West-Enders / Navy in the Thunder Bay junior hockey league where he led the league in scoring two seasons in a row. He also spend three years in the Canadian navy during WW II.
Pentti continued with his success in the EHL where he played for the Boston Olympics. He led all playoff scorers in the 1946 playoffs with 13 goals. The next season (1946-47) he dominated the EHL and scored a league high 49 goals and 92 points in 56 games for the Olympics. He was also the leading scorer in the playoffs with 8 assists and 15 points. His impressive season even gave him the opportunity to make his NHL debut with the Boston Bruins in the 1947 playoffs (one game).
He played in two more playoff games the next season before Boston shipped him to the NY Rangers to complete an earlier transaction involving Grant Warwick on February 6, 1948. As a rookie for the NY Rangers, Pentti was an instant hit,scoring 30 points (14 goals and 16 assists) in 59 games. Although his numbers aren't impressive by today standards it was enough to earn him the Calder Trophy.
The next season (1949-50) Pentti scored 18 goals, but he was more noted for his heroics in the playoffs. He not only led all playoff scorers with 11 points (6 goals and 5 assists) in 12 games but also managed to shut down Montreal's super star Maurice "Rocket " Richard. Richard only managed to score in one of the five games thanks to Pentti's relentless checking of him
Pentti himself managed to score a hat trick in a 4-1 win in game 3. After the series he was dubbed "Lucky Lund" by the Montreal media. He then continued with his fine play in the finals where the Rangers eventually lost in 7 games to Detroit.
His playoff heroics proved to be his last big moment in the NHL. He slumped to just four goals and 20 pts in 1950-51. He was dealt back to Boston with Gus Kyle for Paul Ronty on September 20, 1951. Pentti suffered a serious eye injury during the 1951-52 season that almost ended his career. He rebounded back from the injury and played the entire season in Boston where he scored 17 points (8 goals and 9 assists). Pentti finished his career with the Soo Greyhounds of the NOHA. (North Ontario Hockey Association). where he played until 1955.
Pentti's brother Joe also played professionally for a long time although he did not quite make it to the NHL. After Pentti's career was over he became a longtime sports editor of the Thunder Bay Times-News.
He later split his time between Thunder Bay and Florida.