Around 4.4 million ha of land in USDA Conservation Reserve Program (CRP) contracts will expire between 2013 and 2018 and some will likely return to crop production. No-till (NT) management offers the potential to reduce the global warming costs of CO2, CH4, and N2O emissions during CRP conversion, but to date there have been no CRP conversion tillage comparisons. In 2009, we converted portions of three 9–21 ha CRP fields in Michigan to conventional tillage (CT) or NT soybean production and reserved a fourth field for reference. Both CO2 and N2O fluxes increased following herbicide application in all converted fields, but in the CT treatment substantial and immediate N2O and CO2 fluxes occurred after tillage. For the initial 201-day conversion period, average daily N2O fluxes (g N2O-N ha−1 d−1) were significantly different in the order: CT (47.5 ± 6.31, n = 6) ≫ NT (16.7 ± 2.45, n = 6) ≫ reference (2.51 ± 0.73, n = 4). Similarly, soil CO2 fluxes in CT were 1.2 times those in NT and 3.1 times those in the unconverted CRP reference field. All treatments were minor sinks for CH4 (−0.69 ± 0.42 to −1.86 ± 0.37 g CH4–C ha−1 d−1) with no significant differences among treatments. The positive global warming impact (GWI) of converted soybean fields under both CT (11.5 Mg CO2e ha−1) and NT (2.87 Mg CO2e ha−1) was in contrast to the negative GWI of the unconverted reference field (−3.5 Mg CO2e ha−1) with on-going greenhouse gas (GHG) mitigation. N2O contributed 39.3% and 55.0% of the GWI under CT and NT systems with the remainder contributed by CO2 (60.7% and 45.0%, respectively). Including foregone mitigation, we conclude that NT management can reduce GHG costs by ~60% compared to CT during initial CRP conversion.