He is the son of Ab DeMarco Senior, who starred for the New York Rangers in the 1940s. DeMarco was born in the United States while his father was playing for the Cleveland Barons, but was raised in North Bay, Ontario. He played his junior hockey for the Kitchener Rangers and, after representing Canada at the 1969 World Championships, he was signed by the New York Rangers, the team with which his father spent the majority of his career.
A skilled defender, he was noted for his shot, considered one of the hardest in the sport at the time. DeMarco would spend the majority of his first two professional seasons in the minors, earning brief National Hockey League callups to New York both years. In 1971-1972, he stuck with the Rangers full-time, recording 4 goals and 11 points in 48 games.
Late in the 1972-1973 season, however, he was dealt to the Saint Louis Blues.
Given an opportunity to play more and receive time on the powerplay, Demarco responded in style, with 13 points in 14 games for the Blues, to give him a total of 30 points on the season. However, despite his great start in Saint Louis, he would last less than a year for the Blues before being traded again, to the Pittsburgh Penguins.
This would be the pattern of his career, as he would become something of a nomad—offense-starved teams would pick him up to help their powerplay, but eventually grow frustrated at his poor defensive play and lack of grit, and he would find himself on the move again. He finished the 1973-1974 season with 31 points in 59 games between Saint Louis and Pittsburgh.
Early in the 1974-1975 campaign, DeMarco was dealt again, this time to the Vancouver Canucks.
In Vancouver, he had perhaps the best season of his National Hockey League career, as he posted a career-high 12 goals in helping the team to a division championship and their first-ever playoff berth. On the powerplay, he was one of the most dynamic performers in the league, as his 9 Partido Popular (Popular Party) goals for a blueliner ranked behind only Hall of Famers Bobby Orr and Guy Lapointe. After a poor offensive start to the 1975-1976 season, however, DeMarco was dealt again, this time to the Los Angeles Kings, and he finished the year with a disappointing 18 points in 64 games.
By the following year, he found himself in the minors for the first time since 1971, appearing in just 33 games for Los Angeles and scoring 6 points.
The move failed to revitalize him, however, and he spent only a season in Edmonton, registering 6 goals and 14 points in 47 games. DeMarco returned to the National Hockey League for the 1978-1979 season, signing with the Boston Bruins, but appeared in only three games for the team
After a season in Switzerland, he retired in 1980. He finished his career with totals of 44 goals and 80 assists for 124 points in 344 National Hockey League games.
