At twelve Neilly was already working in the print-fields and he and his elder brother, Dennis, whose son and grandson would also be professional footballers, were lodging with another Irish family at 17, Burn St., Bonhill. However, their mother was not far way. She too was lodging, also alone, recorded as an agricultural worker, and with a third Irish family just down the road at number 5. And the same may have been the case a decade later, with Neilly already away. His mother, still an agricultural labourer was living in another Irish lodging-house. But it was in Renton, which may be the explanation of why in 1885, aged seventeen and perhaps also having moved there, he first turned out for that village's club and not the one nearest to Bonhill, Vale of Leven.  

Neilly McCallum would play for Renton for three seasons, rapidly building a reputation on the right-wing and earning the epithet, "The Shadow". In 1888 he was a member of the team that would win the Scottish Cup against Cambuslang, his first and only Scottish cap and the "Word Championship" versus West Bromwich Albion. But just a week later he was in the first ever Celtic team, defeating Rangers Swifts with Willie Maley and James Kelly behind and Tom Maley and Johnny Madden beside him. And in that game he would score Celtic's first ever goal. 

Neilly would stay at Celtic for two seasons. In that time he was also to score its first Scottish Cup goal before once more hitting the net for the losing eleven in the replayed 1889 Scottish Cup Final and then in February 1890 heading South. He went to briefly to Blackburn, moving on to Nottingham and Forest for the following season. In early 1891 he was in the city, recorded as a professional footballer, boarding with another, John Duncan, also Scots. But he was to return to Scotland in 1891, playing in Celtic's Cup-winning team in 1892.

But he clearly had other things on his mind. Officially for three seasons he was back at Forest but this time with less success. He spent only the first of them at the City Ground. After that he was on loan down the leagues at Loughborough and Newark, which may have had something to do with in 1893 he having married a local girl, Hannah Nuttall, a married that was not a success. Only in 1895 did he settle with a season back in the top flight at Notts County before again dropping once more to Heanor for 1896-7.