I've been on math grad admissions committees before and our school gets a number of applicants from people who've been in the work force for some time. For these applicants I generally look for 3 things: (1) academic background to date, (2) how far they are removed from academic math now, and (3) motivation to pursue grad studies.
(1) is about your abilities, preparation and your work ethic in the past. This is something your former professors/research supervisors discuss and is reflected in your transcript/CV.
(2) is mostly about if you've forgotten too much advanced math to succeed in grad classes. This is reflected in your CV/letters/statement, including how long you've been out, and your GRE score.
(3) is because motivation and work ethic are just as important as ability and preparation to succeed in grad school, and these may have changed quite a bit since you finished undergrad. So I want to see evidence of this in letters from people who have had recent interaction with you, which might exclude your former professors. Also, this should come through in your statement and may be supported by your CV as well.
Assuming your employer can address (3) and possibly (2) in a compelling way (e.g., by giving concrete examples rather than a pro forma generic statement), I would find one letter from an employer quite useful. You should still be able to submit 3-4 additional academic letters, which should be sufficient to discuss your academic experiences, so I don't see this letter taking away anything from your application.