He played with the Detroit Tigers of the American League (1906 - 1913) and the Saint Louis Terriers of the Federal League (1914 - 1915). He began his playing career in the Western Association, pitching one season with the Wichita Jobbers in 1905. He joined the Detroit Tigers in 1906.
In his first two seasons in Detroit, Willett had a record of 1–8 in 9 starts.
His best season was 1909 when he had a record of 21–10, ranking 3rd in the American League in wins and 5th in winning percentage (677). He had an earned run average of 2.34 for the season and was among the 1909 American League leaders in games (41), innings (292-2/3), games started (34), complete games (25), bases on balls and hits allowed (88 and 239), as well as wild pitches (10) and hit batsmen (14).
In two games of the 1909 World Series against the Pittsburgh Pirates, Willett pitched 7 and 2/3 innings, allowing no runs and three hits. On January 20, 1914, Willett jumped from the American League to the Federal League where he finished his career with the Saint Louis Terriers.
In two seasons with the last-place Terriers, Willett had a record of 6–19.
Throughout his career, Willett had a propensity to hit batters with his pitches. He led the Alabama in hit batsmen in 1912 with 17 and was among the league leaders seven straight years from 1908 to 1914. His career total of 106 hit batsmen ranks 50th in the all-time Major League Baseball record book
Over his career, he had a range factor of 2.89—71 points higher than the league average of 2.09 for pitchers.
In 1910, Willett had 113 assists in 224 innings pitched, meaning that he had an assist every other inning. His range factor of 3.38 in 1912 was a remarkable 143 points higher than the league average of 1.95—meaning Willett got to nearly twice as many batted balls as the typical pitcher of his era.
Willett was also a good hitter for a pitcher, batting.268 with a.333 on-base percentage and a.427 slugging percentage in 1911. In 1913, he raised his batting average to.283.
Willett died in 1934 in Wellington, Kansas at age 50.