Mack's lonely journey, which began on March 23, took him through 52 countries over five continents. To conform with Guinness World Records requirements, the route crossed the equator twice.
"It was supposed to take between two to three months and it's been five months now," he told The Associated Press. Administrative formalities in Crete and Dubai "because of paperwork issues, visas, permits, things like that," caused the delay.
The flight took him through Africa and the Gulf region — where he face periods of extreme heat — then on to India, China, South Korea and Japan. From there, he headed to Alaska and down the US West Coast to Mexico.
The teen then headed north again along the US East Coast to Canada, across the Atlantic via Iceland, to the UK and Belgium.
If all goes well, and weather permitting, he'll fly east across Europe via Slovakia and land at an airport in the Bulgarian capital, Sofia, on Wednesday.
Proud father Sam Rutherford said his two children have set a shining example.
"They have got around the world safely, effectively, professionally. And they've shown to other youngsters that you don't have to be 18 even, and certainly not 30, to make a difference and do something and follow your dreams," he told AP.