Taking over from Ian Brightwell in late February, Alexander has steered the Cheshire club clear of relegation with an impressive 17-point haul in League Two.
However, his latest win was against the balance of play with Chesterfield wasting a succession of chances at Moss Rose on Saturday.
In the fourth minute, Janos Kovacs saw a 15-yard header cleared off the line by Neil Ashton before Jamie Ward blasted an 18-yard volley straight at goalkeeper Jon Brain.
Lloyd Kerry was the next to go close forcing an acrobatic save from Brain before Graeme Owens beat two men down the left flank only to be thwarted by the alert Brain.
And Chesterfield were made to pay in the 40th minute when Ashton picked out youngster Gareth Evans who slotted home his seventh goal of the season from the six-yard line. Evans went close late in the second half but was denied by Spireites goalkeeper Barry Roche.