Neagle grew up in Dimboola, a country town in western Victoria. There he formed a lifelong friendship with future teammate Tim Watson. They played junior football and basketball, and shared a paper round.
Neagle joined Essendon, in 1976, preceding Watson who joined the club the following year as a 15-year-old. Both players made their senior debuts in 1977. Neagle finished second in the 1980 Brownlow Medal and equal fifth in 1981.
Neagle was selected to play for Essendon in the 1983 VFL Grand Final. He also played in the 1984 VFL Grand Final, where Essendon won their first premiership in 19 years. Neagle kicked the last goal of the match.
Neagle missed the 1985 premiership. He played in the second semi-final win over Hawthorn, but was a late withdrawal on Grand Final day due to a leg injury.
At the end of the season Neagle transferred from Essendon to Sydney, where he played in 1986 until his retirement.
Neagle was offered a rural coaching position in 1989, but chose to remain as a player with Sydney. In all, he played five seasons for Sydney before retiring at the end of the 1990.
After retiring from AFL football, Neagle coached and played at a number of clubs around Australia, including Merbein and Sale in Victoria, Mount Barker Football Club in the Great Southern Football League (GSFL) and Balranald Football Club in the Central Murray Football League (CMFL). In 2009 he coached St Mary's to the Northern Territory Football League (NTFL) premiership. He also coached North Albury and Mangoplah-Cookardinia United-Eastlakes in NSW, and in 2012 Walla Walla Football Netball Club in the Hume Football League.
Neagle worked as a truck driver and was the father of former Essendon player Jay Neagle, who was drafted under the father-son rule at the end of the 2005 AFL season.
Neagle died when the trailer of the B-double truck he was driving flipped over and crushed the cabin at Tharbogang, near Griffith, New South Wales.
Neagle's death at 54 years of age prompted a flow of tributes from former teammates and associates. Coach Kevin Sheedy recalled that Neagle:
...was not only a brilliant footballer but was also a real character. Merv was a very tenacious, hard-running player and his courage and toughness were typical of Essendon teams of that era. He had great physicality for a wingman and was a terrific kick for goal on the run. He also gave a lot back to country football after he retired, and along with Tim Watson was one of the finest players to come out of Dimboola. Merv has left us all too soon but has left those who knew him with many great memories.
Essendon teammate and premiership captain Terry Daniher talked of the shock that he and his teammates felt on hearing the tragic news and added:
He was a terrific hard-running player on the wing for us, who gave us his all. He was a great mate who loved a beer and loved to be around his teammates. He was just a good bloke.
Lifelong friend Tim Watson described Neagle as a likable larrikin who was a great footballer in one of Essendon's golden eras. One of his favourite memories of Neagle was his performance during the 1984 second semi-final against Hawthorn:
That second semi he played in 1984 against Hawthorn was against Dipper and it was one of the greatest man-on-man contests in the last 50 years. They just went hammer and tong on the wing when blokes used to play on each other. They were both tough and aggressive.
Neagle's employer at Hehir's Transport Ross Hehir said he was a fine employee who was on his second stint at the company:
He would have known a squillion people and there wouldn't have been any who'd have a bad word to say about him.