The european kingdom which took the early 15th century lead in overseas exploration was

Portugal is the European Kingdom that led in European  Exploration in the 15th Century in the age of discovering, exploration and mapping coasts of Africa, Canada, Asia and Africa. Under the leadership of Prince Henry the Navigator, Portugal took the role to search for a sea route to Asia by sailing south around Africa, and during this process, Portugal accumulated wealthy knowledge on the geography and navigation of the Atlantic ocean.
In the last decade, Christopher Columbus crossed the Atlantic Ocean and  "discovered" a new continent while searching or a sea route to the Indies.
In 1498, Vasco da Gama led the first fleet around Africa to India.