After college he signed a contract with Nike and won his first national title at the 2003 USA Outdoor Championships. Slattery made his senior international debut at the 2003 World Championships in Athletics and ran a personal record time of 8:22.32 minutes in the heats. He failed to build on this success the following year and missed a spot on the Olympic team after finishing fifth at the United States Olympic Trials. In 2005 he set a personal record time of 8:17.87 minutes at the FBK Games, the fastest by an American that year. A third-place finish at the USA Outdoors brought him his second world championship appearance, although he was again eliminated in the heats at the 2005 World Championships. In 2006 he represented the United States in the 3000 meters flat at the 2006 IAAF World Indoor Championships – Men's 3000 metres and in the steeplechase at the 2006 IAAF World Cup. He was the runner-up at the usatf national championships that year and his season's best time of 8:22.70 minutes was second in the American rankings.