Short answer. It can be valid, but only under specific conditions. Substituted service at your office is allowed by leaving copies with some competent person in charge thereof, and only after personal service failed after at least three attempts on two different dates for justifiable causes.
What the law says
By leaving copies of the summons at [the] defendant's office or regular place of business with some competent person in charge thereof. A competent person includes, but is not limited to, one who customarily receives correspondences for the defendant
Rule 14, Section 6 — Substituted service. Read the full provision →
Substituted service is not the default option
This method only becomes available if, for justifiable causes, the defendant cannot be served personally after at least three attempts on two different dates. Substituted service at your office is a fallback, not a starting point. If the sheriff never made the required attempts to serve you personally first, on the required number of separate dates, service at your office would not satisfy this rule's own preconditions for resorting to substituted service in the first place. This precondition binds the sheriff and the plaintiff, not you — you are not required to prove they failed, only to raise it if the record shows they never really tried.
Whether your coworker qualifies matters as much as where they were
The rule requires the summons be left with some competent person in charge of the office or regular place of business, and it gives an example: one who customarily receives correspondences for the defendant. Not every coworker automatically fits that description — someone with no responsibility over your correspondence or office affairs, and no real authority there, may not be the 'competent person in charge' this rule actually contemplates.
Why both conditions have to line up
Substituted service is valid only when its specific preconditions are actually met — the failed personal-service attempts on your part, and a person genuinely in charge of your office receiving the copies. If either piece is genuinely missing, whether the required attempts actually happened first, or whether the coworker truly counts as competent and in charge, the substituted service could reasonably be open to challenge on that basis.
What to check about your own situation
Since the answer turns on facts specific to your case — how many attempts were actually made, on how many dates, and what role your coworker actually has at the office — those details are what determine whether this particular substituted service met the rule's requirements, rather than any general assumption that simply leaving papers somewhere at a workplace is automatically sufficient all by itself.