Sullivan was originally selected by the Reds in the 29th round of the 2014 draft out of Gloucester Community College, where he was nearly unhittable. After dominating the rookie league level in 2014, Sullivan had another solid season in Low-A in 2015. In 2014, his K/9 was off the chart at 13.3. It did drop to the low seven range at Low-A, but his stuff has still be impressive. He has been used primarily as a reliever, and walks and hits were up through May in Low-A in 2016 in a repeat of the level.