Leonard, 25, underwent a medical in South Yorkshire last night after reiterating his desire to join Chris Wilder’s side.
The Star understands personal terms have already been provisionally agreed with the former Plymouth Argyle youngster, who could make his United debut against Sheffield Wednesday next week.
Wilder hopes to complete at least one more new signing before Friday’s Steel City derby although budgetary constraints are likely to prompt the departure of Ben Heneghan and Nathan Thomas on loan. James Hanson has already completed a temporary switch to Bury.
United, who face Ipswich Town in the FA Cup today, have paid around £700,000 to secure Leonard’s services.
Fleetwood Town centre-forward Devante Cole is not thought to be an immediate target, despite holding talks in August.
“We are trying, but it’s not an easy market to work in,” Wilder said. “Everybody recognises we haven’t got an open chequebook.
“We can’t just blow anybody out of the water, players we are after are good players and are interesting other clubs as well.
“We have to work hard to get players through the door at this football club, as we always have done, because it’s not been the case of a numbers game. It’s tough, and been tough for 18 months, in terms of players we want to get in.”
Last night’s announcement concludes the longest running transfer saga of Wilder’s 20 month reign, after United first expressed their interest in Leonard during the close season.
Although Southend refused to accept their initial bid, which was believed to involve a series of staged payments, a combination of Wilder’s persistence and Leonard’s determination to push through the move eventually forced the League One club’s hand. Indeed, a spokesperson for Roots Hall, admitted it was his “dream” to join United after nearly seven years in Essex.
Leonard, who can also operate at centre-half, made 265 appearances for Southend after leaving Plymouth.