Neill was an integral member of United's team that narrowly missed out of promotion to the Championship last season but his will and leadership skills are expected to be prominent again during this campaign.
He was United's only capture of January 2011 transfer deadline day, with the Blades paying Leeds an undisclosed fee for his services.
The combative central defender played two games during a loan spell with United in 2006, including a Steel City derby win at Hillsborough.
He began his career with Queens Park and also represented Dumbarton before heading south of the border with spells at Sunderland, Wolves and Preston North End.