I know it would never happen…But could he?
Per this article from the Christian Science Monitor, presidents Reagan and Bush were summoned while still president (but deferred service), Bill Clinton was summoned after leaving office (but dismissed by the judge) and Rudolph Giuliani served on a jury while mayor of New York City.
Not sure why he would be excluded be default. If he asked to not show up they would excuse him. I believe we had a local office holder (city council) who served on a jury.
Not quite the Presidential level, but Supreme Court Justice Stephen Breyer was called to serve on a jury in 2005 (for a drunk driving case). One of the lawyers ended up requesting he be excused.
Often exemptions can be made automatically for those who are summoned, but their job provides a ‘critical’ service to the community. Usually that includes people who already work in the legal system, emergency workers, medical workers, and sometimes politicians.
I would imagine that anyone remotely sane would acknowledge that being the President is a rather critical job. And even if they didn’t, I have no doubt that the judge and both lawyers would be tripping over themselves to dismiss the President, even if only because it would most certainly turn the trial into an absolute circus, and thus be a huge pain.
Apparently even The Royal Family are no longer on the list of those exempt from jury service here in Britain …
I can only assume it was the lawyer who figured he had the losing case. 
Actually, I believe lawyers are rarely allowed to serve on juries.
Not in England and Wales after the Criminal Justice Act 2003. Now pretty much everyone serves and Crown Prosecution Service lawyers have been called as has one retired law lord.
Ordinarily, yes, but there are so many lawyers in Washington DC that this is no longer true here.
In Ohio, virtually all career exemptions from jury service were abolished years ago. Judges, magistrates, lawyers, police officers etc. all can and do serve on Ohio juries, subject of course to dismissal for cause or peremptory challenge. Sometimes the presiding judge will give a limiting instruction or get an assurance from the prospective juror that he or she will not dominate jury deliberations by virtue of his or her background or training.
The President is registered to vote and drive in Illinois. Presumably, if summonsed in Illinois, he would be excused after he reminded the court that he is now a fulltime temporary resident of Washington, D.C., and that serving on a jury in Illinois would cause a hardship. He would not appear on voter or driver registration rolls in Washington itself, and thus is very unlikely to be summonsed to serve on a District of Columbia jury.
Due to his important, time-consuming and weighty public duties, I would unhesitatingly excuse the President from serving on any jury over which I was presiding. If everything fell into place, however, I know of no legal reason why he could not be a juror. Indeed, it might send an important and civic-minded message to his fellow Americans if he did.
My recollection is that Mr Bush was summoned by a Texas court while he was serving as President. He appeared by agent and requested a deferral until he was no longer in office, which the judge granted.
found the article: Jury duty is for Everyman.
as well, there was a similar episode while he was Governor of Texas. Mr Gonzales appeared for him and got him excused, in an episode that was not without some controversy, since it may have shielded Mr Bush from revealing his own drunk driving conviction: Gozales’ Truthfulness Long Disputed.