Wallsend-born Smith, who started his career with Darlington, did not celebrate his early goal in front of the United supporters but would surely have enjoyed netting against his heroes, but Hendrick's goal was the least United deserved after recovering well from a slow start to the game.
It was also a memorable evening for teenager Joe White, who made his Magpies debut as a late substitute on a night which saw 21 players get game time for United, including Fabian Schär and Emil Krafth, who played their first minutes of pre-season after being given an extended break following their involvement in Euro 2020.
Allan Saint-Maximin missed out for United with illness while Joelinton was injured, but Freddie Woodman returned in goal having followed protocol after being identified as a close contact of Karl Darlow, who tested positive for Covid-19 earlier in the month, and Matt Ritchie was also back in from the start after a knock.
The Millers included former Magpies midfielder Dan Barlaser in midfield, but new signing Shane Ferguson was not part of Paul Warne's squad against the club where he started his career.
Smith's goal came in the ninth minute when a fantastic left-wing cross from Mickel Miller was emphatically headed home by the former Wallsend Boys Club frontman.
He then headed wide at the far post from another Miller delivery, this time from the right, before Miller was involved again with a dangerous low cross which Freddie Ladapo just failed to force over the line.
United began to gain a foothold in the game in the midway point of the first half, and after Dwight Gayle was fouled right on the edge of the box, Ryan Fraser played a short free kick to Sean Longstaff but his long-range shot was off target.
Then Ritchie's cutback found Longstaff but his first-time strike went over the bar. And as half time approached, Newcastle fashioned arguably their best opportunity yet as Callum Wilson saw his effort flash just wide of the far post.
Steve Bruce made four changes at the break and one of the new arrivals, Jacob Murphy, wasted no time in making an impact with a strong run and inviting cross which Gayle turned goalwards but the Millers managed to scramble the ball clear.
Then Ritchie's excellent free kick was met by the head of Federico Fernández but Viktor Johansson produced a superb save from point-blank range.
The Rotherham goalkeeper then made a more routine stop to hold a Longstaff strike from the edge of the box, before a jinking Gayle run ended with a low shot which whistled just the wrong side of the far post.
But, with time running out, substitute Hendrick rose highest to connect beautifully with Murphy's corner and equalise for the Magpies.
Newcastle United: Freddie Woodman (Mark Gillespie 46), Ciaran Clark (Paul Dummett 46), Federico Fernández (Emil Krafth 65), Jamaal Lascelles (c) (Kell Watts 46), Jamal Lewis, Ryan Fraser (Jacob Murphy 46), Isaac Hayden (Matty Longstaff 73), Sean Longstaff (Jack Young 80), Dwight Gayle (Joe White 77), Callum Wilson (Jeff Hendrick 73), Matt Ritchie (Fabian Schär 65)
Subs not used: Dan Langley, Matt Bondswell
