2010-11: Ryan Mantha skated in four games with the Little Caesars midget team as a 14-year-old. He had no points with 2 penalty minutes. In 28 games with the Little Caesars bantam team he scored 4 goals with 10 assists and had 29 penalty minutes.
2011-12: Mantha played forward and defense for the Belle Tire U16 midget minor team in Michigan. In 38 games he scored 8 goals with 14 assists. He had 1 assist with 19 penalty minutes in six playoff games. Mantha was selected by Sault Ste. Marie in the fifth round (97th overall) of the 2012 OHL Priority Draft and signed a tender offer with Sioux City prior to the 2012 USHL Future's Draft in May. In August 2012 he committed to playing college hockey at North Dakota in 2014-15.
2012-13: Mantha, playing defenseman, skated in 52 games for Sioux City in his first USHL season. He scored 1 goals with 6 assist and was +15 with 48 penalty minutes. The Musketeers finished sixth in the Western Conference; missing the USHL playoffs. Mantha was invited to USA Hockey's Select 17 Player Development Camp.
2013-14: Mantha began the USHL season with Sioux City before being traded to Indiana in January. He represented the USA in the 2013 Ivan Hlinka Memorial and 2013 World Junior A Challenge tournaments. Mantha had 1 goal with 5 assist and was an even plus/minus with 51 penalty minutes in 29 games with the Musketeers before joining Eastern conference-leading Indiana. Playing with the USA U19 team at the WJAC he was scoreless with 16 penalty minutes in four games. Mantha had 1 assist and was +2 with 27 penalty minutes in five games for the USA U18 team at the Ivan Hlinka Tournament. He skated in the USA Hockey/CCM All-American Prospects Game in September and was ranked 179th amongst North American skaters in Central Scouting's mid-term rankings prior to the 2014 NHL Draft.