A former £1m player from his time with Coventry City, McKenzie was scoring goals for the Sky Blues in the Championship as late as February last year before being stopped in his tracks by an ankle injury which has hampered his career ever since.
Northampton boss Ian Sampson however has run the rule over the 32-year-old in training and been given the go-ahead to do a deal following his side's plum League Cup draw away to Liverpool.
Sampson told the club's website: "He is an experienced striker with a good goal-scoring record playing at a higher level.
"He has got an excellent CV and a good pedigree and he has a hunger to get back among the goals."