On 5 September 1870, he joined Charles Todd's party to build the central section of the Overland Telegraph Line. On 7 March 1871, Mills and his companion Gilbert McMinn, having found a gap in the McDonnell Ranges through which to feed the telegraph line, and which he named Heavitree Gap, then came across a string of waterholes along what is now called the Todd River, which were already frequented by local Arrernte people. Mills named one Alice Spring, after the wife of Charles Todd, from which the town of Alice Springs now takes its name.