A strong left-handed pitcher, Mehallow was drafted by the California Angels in 1980 while playing for Belleville Area College.

He pitched for the Sauget Wizards from 1987 to 1990, guiding his team to the Mon-Clair League championship in 1988 and 1989. The Wizards won the Metro East Night League championship in 1989, and also defeated Team USA. Mehallow was the starting pitcher against Team USA in 1989 and 1990. During his four years with the Wizards, he won 57 games and received three ERA awards.

From 1984 to 1986, Mehallow pitched for the O'Fallon Merchants in the Mon-Clair League. In 1984, he threw two no-hitters and had a streak of 35 consecutive scoreless innings. He won the ERA title in 1986.

From 1979 to 1983, he pitched for the Shiloh Blue Sox in the Mon-Clair League.

During his twelve years in the Mon-Clair League, he won numerous MVP awards and played on nine All-Star teams.

In 1999, Mehallow was inducted into the Mon-Clair League Baseball Hall of Fame.

He starred for O’Fallon Township High School in the late 1970s. In 1979, he was named to the Mississippi Valley All-Conference team and the Post-Dispatch All-Metro team.

He was an all-star pitcher for the O'Fallon American Legion team in 1978 and 1979.

He pitched for Belleville Area College in 1980 and 1981, Florida Southern College in 1982, and McKendree College in 1983.

Mehallow managed the O'Fallon American Legion team in 1983 and 1984.