Can a sonic boom arrive before the source?
Assuming the aircraft you saw was traveling in a straight line, then the sonic boom you heard (assuming it was a sonic boom) could not have been generated from that same aircraft.  This is because the sonic boom is experienced at an angle back from the flight path of the aircraft.



Many aircraft which are capable of generating sonic booms also happen to fly in groups.  Fighter jets are a good example of this.  Fighter jets often train in groups of more than one aircraft.

It's possible you may have heard a sonic boom from another aircraft at a higher altitude which you did not see, then you later saw a new aircraft.  This is actually very likely, because sonic booms are much more common at high altitude.  The atmospheric conditions for creating sonic booms are favorable at high altitude.  This is because the speed of sound is much lower at high altitude compered to the speed of sound at low altitude.  It is difficult to see military aircraft at high altitude against a blue sky background because the military intentionally paints the skin of the aircraft in a grey color to make it difficult to see against a blue sky.  So without a con trail you likely did not see the fighter jet which was flying at high altitude.

Other commenters asserted supersonic flight is not allowed inside the US, but this is not entirely correct.  Many restricted airspaces in the US allow supersonic flight.  The restrictions on supersonic flight are generally put in place to balance the concern of noise pollution for the population.  Some MOA (military operating area) also allow supersonic flight, although usually only at high altitude.  Some areas of the eastern US which allow supersonic flight require the pilots to fill out a log stating supersonic flight occurred.  However, supersonic flight is easy to do accidentally in many fighter aircraft.  The pilot may not realize he was supersonic until reviewing the tapes after the flight.  Most airspaces allowing supersonic flight are over sparsely populated deserts of the western US.   

Picture credit to Wikipedia