Mangus's initial career was in the family dairy farming business, Mangus Dairy Farms. He became the president of LaVille Farms, Inc. as well as a director of Mangus Dairy Farms, Inc. Related activities included the Pure Milk Association, the local Farm Bureau, American Milk Producers Association and St. Joseph County Extension Advisory Board. He was also a professional truck driver. After part of his family's farm was taken during expansion of U.S. Route 31, Mangus became politically active, especially in the local Union Township Republican Party. He became a Republican state convention delegate in 1964 and 1966. During the Republican landslide of November, 1972, Mangus defeated three-term Democrat Clayton Joseph "Joe" Barber to become the delegate from southern St. Joseph County in the Indiana House of Representatives. Mangus won re-election many times, and ran unopposed during the last several elections, before announcing his decision not to run again because of ongoing health issues. He rose to become the chairman and/or ranking member of the House Environmental Affairs Committee, the Constitutional Law Committee, and the Elections and Reapportionment Committee. He also served on the HouseHuman Affairs Committee and House Public Policy Committee. During his lifetime, Mangus received awards from the Fraternal Order of Police, Professional Firefighters, Soil and Water Conservation District and Junior Chamber of Commerce, among others. Mangus became known for his advocacy of environmental causes, particularly the creation of Potato Creek State Park in 1977 with the help of Indiana Governor Otis Bowen. Mangus helped oversee Potato Creek's upgrade to full state park status in 1983. He also served on the Department of Natural Resources Commission Although an "ordinary guy", Mangus also became known for his knowledge of parliamentary procedure, in particular for proposing a final amendment to strip prior approved small amendments and return a bill to its original form, which some called the "Mangus Maneuver". In 1995, Mangus used the opposite maneuver to block proposed reform of Indiana's time zone problem through "silly" amendments. After Mangus declined to run for re-election in 2004, fellow Republican Jackie Walorski succeeded him, and would also serve multiple terms in the Indiana House before eventually winning election to the U.S. House of Representatives.
Mangus died of a heart attack on February 4, 2008 at Memorial Hospital in South Bend, St. Joseph County, Indiana. Many Indiana legislators of both parties attended his funeral at the County Line Brethren Church. During his life, Mangus thrice won the Sagamore of the Wabash award. He is interred at Fair Cemetery in North Liberty, Indiana. Indiana Route 31 near Potato Creek State Park and Lakeville is named in his honor. Potato Creek State Park displays a plaque and an explanatory marker near its entrance noting his role in the park's founding. Bethel College has a dormitory honoring Russell E. Mangus, who helped found the college in 1947 and is not a direct relative, although similarly a dairy farmer from Union Township, St. Joseph County, Indiana who advocated soil conservation practices and was active in the Evangelical United Brethren Church.
