People in Power
Longest Serving National Leaders
As the below infographic shows, Fidel Castro, who retired from his reign over Cuba in 2011 due to ill health, was the longest serving non-royal national leader so far. The second longest serving leader is Kim Il-sung who ruled over secluded North Korea for more than 48 years. The discussion page on Wikipedia notes that: "It appears that this page has been edited at least once to make Kim Il-Sung a current ruler for North Korea, still accruing time at the top of the list despite having died in 1994 because he is 'Eternal President of the Republic.' That's what you call a personality cult.

Description
This chart shows the time in power of longest serving non-royal national leaders since 1900