Watford have been talked up as potential promotion contenders after a bright start to the Championship season under new boss Gianfranco Zola, but they were given a harsh lesson at Pride Park.
Rams skipper Richard Keogh headed his side in front and Watford's vulnerability in the air was evident once again when midfielder Jeff Hendrick headed home a second goal.
Sammon was a handful in his first home appearance for Nigel Clough's side since his £1.2million move from Wigan and he added a third goal just before half-time.
Highly-rated midfielder Will Hughes added Derby's fourth goal just after the break, his first goal for the club, and the lively Jamie Ward made if five with his first of the season.
Matej Vydra grabbed a consolation for the Hornets, but the damage had already been done and the victory could have been even more comfortable for the Rams.
After a cagey start Derby started to get into their stride and they took the lead after 15 minutes when Ward swung in a free-kick which skipper Keogh headed home from six yards out.
Derby doubled their lead after 36 minutes when Jonathan Hogg fouled Sammon and Paul Coutts delivered an inviting free-kick which Hendrick rose at the back post to head beyond Manuel Almunia.
Sammon almost opened his Derby account before the break but the big striker's shot was deflected away for a corner.
But former Wigan striker Sammon was not to be denied and he added Derby's third goal two minutes before half-time, outmuscling Carl Dickinson and producing a delightful chipped finish over Almunia to leave Watford reeling.
Keogh almost added a fourth goal on the stroke of half-time when he rose to meet Coutts' free-kick but his header struck the top of the crossbar.
That was a let-off for Watford, but they were four goals down four minutes into the second half.
Former Arsenal keeper Almunia produced a fine save to keep out Ward's free-kick and England Under-17 midfielder Hughes was on hand to tuck home the loose ball off the post from close range.
There was worse to come for Zola's side as they conceded a fifth goal in the 68th minute, with Coutts once again the architect as he won the ball inside the penalty area and picked out Ward, who made no mistake with his left-foot finish from 12 yards out.
Two of Watford's summer signings combined to get the visitors on the scoresheet after 73 minutes when Ikechi Anya picked out Vydra, who slotted home with his right foot.
But Derby were out of sight by then and they have now scored 12 goals in their first three home games of the season.