Siwy started his 1981 campaign with the Single-A Midwest League's Appleton Foxes. He was named to the league's All-Star roster that year with a 5–0 record and a 1.96 earned run average. This earned Siwy a call up to the Double-AGlens Falls White Sox of the Eastern League where he went 11–4 with a 3.85 ERA. Invited to spring training in 1982, Siwy was assigned to the Triple-AEdmonton Trappers of the Pacific Coast League for much of the season. Over the course of the season he would earn a 12–8 record with a 4.04 ERA. He earned his first call-up to the Major League roster in late August. Debuting in relief on August 20th against the Kansas City Royals, Siwy struck out the first batter he faced, John Wathan, but would later allow a three-run home run to Hal McRae and two other runs before being removed. All told, in two innings of work, Siwy allowed four hits, five earned runs, and walked three batters. Three days later, against the Cleveland Indians, Siwy was given a start by the White Sox. Over five innings, Siwy would allow six hits and three earned runs. He received a no-decision in the game. Siwy spent 1983 with the Denver Bears of the American Association, which replaced Edmonton as Chicago's Triple-A affiliate. Pitching mostly in relief, he recorded a 6.14 ERA and five saves. His season ended prematurely when he broke two knuckles attempting to punch a chair in anger following a bad appearance in a game. Staying with Denver for the following year, which changed its nickname from the Bears to Zephyrs in the interim, Siwy returned to a starting role at the minor league level in 1984. His time in Denver was interrupted by a major league call-up in May. Siwy made one appearance with the White Sox during his call-up, pitching innings in relief of LaMarr Hoyt on May 12th. He allowed three hits in his outing and two walks, only allowing one run on a Billy Samplesacrifice fly. He was sent back to Denver shortly afterwards. Overall with the Zephyrs, he had a 4–5 record and a 5.25 ERA. On June 26, Siwy was traded to the Cleveland Indians to complete a trade for pitcher Dan Spillner.
Later professional career
Assigned to the Triple-A Maine Guides of the International League by the Indians, Siwy pitched in only seven games with the club in 1984 due to a shoulder injury; when healthy, he recorded a 2.04 ERA and a pair of saves in seven games pitched. He was added to their 40-man roster after the season ended. During the offseason, Siwy played in the Caribbean Series with the Venezuelan Tiburones de La Guaira as a means of working on his pitches. Siwy started the 1985 season as a candidate to make the major league roster. Despite a good spring, he was sent back to Maine, where he would struggle during the first weeks of the season. Siwy was then released by the Indians on April 22nd. He was quickly re-signed by the White Sox and assigned to the Buffalo Bisons of the AA; the team had replaced Denver as the White Sox' Triple-A affiliate for the 1985 season. Siwy's time in Buffalo did not last the season, as he would be released after 16 games, 13 in relief, with the Bisons. He finished the year with the Double-A Birmingham Barons of the Southern League, an affiliate of the Detroit Tigers. Siwy spent 1986 with the San Diego Padre-affiliated Las Vegas Stars of the PCL. He received his highest number of starts in four years, and ultimately went 6–4 with a 3.56 ERA during the season. Siwy remained with Las Vegas for the 1987 season and served predominantly a reliever that year. He finished the season with a 6–8 record with a 5.78 ERA in what would ultimately become his last year in professional baseball.
Later life
Siwy returned to law enforcement in 1989 when he became part of the Las Vegas Metropolitan Police Department. He remained with the LVMPD until retiring at the end of 2012. At the time of his retirement, he was a sergeant for the LVMPD.
