Stennett was born in Colón, Panama, on April 5, 1949.[1] He was raised in the Panama Canal Zone and attended Paraiso High School, the same as Rod Carew. Stennett piqued the interest of the New York Yankees, San Francisco Giants, and Houston Astros, who wanted him to continue his schooling in the United States and develop him into a pitcher. However, he rejected their overtures on the recommendation of his father.[2] He was signed as an amateur free agent by the Pittsburgh Pirates on February 12, 1969.[1]
In his first three seasons with Pittsburgh, Stennett was used at shortstop and second base. He also played a solid defense at all three outfield positions, with an average arm and great reaction speed.[2] He showed progress in 1973, when he hit 10 home runs and 55 RBIs in 128 games.[1] Following the 1973 season, Pittsburgh traded incumbent second baseman Dave Cash to Philadelphia and gave Stennett the starting job.[2] Batting from the leadoff spot,[2] he responded with a .291 average, 84 runs, 56 RBI, and a career-high 196 hits.[1]
On September 16, 1975, Stennett became the only player in the 20th century to have seven hits in seven at bats in a nine-inning game, as Pittsburgh routed the Cubs, 22–0.[10] Stennett's first hit in that game came off starter Rick Reuschel and his seventh was off Rick's brother Paul.[11] Pittsburgh also set a major league record for the largest winning score in a shutout game in the modern era[11] (later matched by the Cleveland Indians in 2004).[12] He was the third player to collect seven hits in a single game, and the second to do it in a nine-inning game,[13] equaling the record set by Wilbert Robinson of the 1892 Baltimore Orioles.[14] With Stennett's position at second base secure in a lineup loaded with young hitters such as Dave Parker, Richie Zisk, and Rich Hebner and complemented by veterans Willie Stargell and Manny Sanguillén, Pittsburgh traded up-and-coming second baseman Willie Randolph to the New York Yankees after the 1975 season.[2]
On August 21, 1977, Stennett was batting .336 for the season, but he broke his right leg while sliding into second base in a game versus San Francisco.[15] He was out for the year and had fewer than the required number of plate appearances (12), falling short of qualifying for the batting title, won by teammate Dave Parker (.338).[14] In that season, Stennett collected a career-high 28 stolen bases.[1] Stennett was part of the 1979 Pirates team that won the World Series. He singled in his only at-bat in the series, in which Phil Garner was the starter at second base for each game.[14]
Stennett signed a five-year, $1 million free agent contract with the San Francisco Giants on December 12, 1979.[16] However, the Giants would release Stennett in April 1982, with three years remaining and $2 million left on his contract.[2] He hit a combined .242 with San Francisco.[14] Not yet 31 years of age, Stennett would never again play in Major League Baseball.[1][2] He played in the Mexican League in 1982 before finishing his professional career with 55 games for the Double-AWichita Aeros in 1983.[3][14]