If I had a lawyer when I received pre-trial diversion, do I need him in order to get the case dismissed after I do my probation time?

The short answer is no.  However, you may want to have an attorney involved with your dismissal.  You mention that you were on a pre-trial diversion.  Pre-trial diversion is different than deferred adjudication or a strait probation, even though all three generally involve reporting to a probation department.  Many district attorney's offices will do the dismissal automatically after you complete a pre-trial diversion.  If you were on a deferred or a strait probation, many probation departments or DA's offices will draft orders discharging you after you have completed the programs.  So, generally, you do not need an attorney to get the case "closed" or dismissed.  The reason you may want an attorney involved is to review the language of the final order.  How that last order is worded can affect you later.  For example, if the DA's office dismissed the pre-trial diversion "without prejudice," they may be able to refile the case later.  If you have immigration issues and the dismissal is conditional, meaning it reads something like, "dismissed because defendant completed pre-trial diversion program," then immigration may still use the pre-trial diversion against you.  If the dismissal was just an outright dismissal (no because language), then immigration is less likely to use the case against you.  The same applies to expunction options.  You'll have a better chance of getting a case expunged if the case was dismissed outright, rather than because of a condition.  Once that final order of dismissal is signed, it's final... which means that you cannot change it later.  You have one shot at getting good wording that will help you clean up your record later.  Consult with a criminal attorney to make sure you don't miss a valuable opportunity.