The 23-year-old has made rapid progress over the last five years, going from non-league football to the UEFA Champions League and the senior England squad.
United moved for his services in 2010, after seeing him catch the eye at Fulham, and he has developed steadily throughout his time at Old Trafford.
Smalling concedes that he could not have imagined his career to take such a path, but insists he will not be taking anything for granted and will keep pushing for continuous improvement.
He told the Manchester Evening News: "Every kid has got that dream to play at this level when they start playing football, but it wasn't a dream I really thought would come true.
"I managed to play my football and I managed to get the odd trial when I was finishing school and luckily it paid off, but I couldn't have dreamed of being where I am now.
"I think it shows anyone who gets into the game that it is never too late.
"It also means there is a lot more from me to come because I've only been in the game for a few years and I'm just striving for more."
Smalling feels he could not be in a better place to work on his game, with it his intention to keep learning off the pool of world-class talent he has around him at United.
"I think it's good for me to be able to learn from Rio Ferdinand and Nemanja Vidic," he added.
"These are invaluable years.
"Hopefully, one day when I become established then people can look up to me, but these are the years that I'm taking advantage of now."