"A GOOD LEADER MUST BE A GOOD MANAGER. A GOOD MANAGER MAY BE NOT A GOOD LEADER". Do you agree with this?

Hi Diem, thank you for your thought provoking question that will sharpen all of us in our understanding of being an effective leader. 

Brian has already given us a very clear, concise & I believe right definition of leadership & management. Peter Drucker, who is a major influence in my leadership journey, also defines leadership & management quite similarly: 

“Management is doing things right; leadership is doing the right things.” 

Brian also made an important comment that an effective person needs both leadership & management skills which I believe is right, especially more so for leaders.

I agree completely with your statement. Good leaders must be good managers while a good manager may not be a good leader. Just 1-2 years ago, I would have disagreed but my position has evolved. Here is why:

Theoretically, there is a seductive danger to separate leadership from management. Peter Drucker, the most influential management theorist of modern times, observes that management is a fairly recent development. In the early 1850s, the largest manufacturing company in the world (a cotton mill in Manchester) employed less than 300 people. (Now prior to this, there have been powerful individuals owning large number of human slaves but that is not management. That is slavery.) As our organizations became larger, it required the development of both management as a task & profession. Drucker explains "In less than 150 years, management has transformed the social & economic fabric of the world's developed countries." 

So management is a fairly new concept & it is very tempting to theoretically separate it from leadership. This led to my belief in the past, that I now believe to be incorrect, that leaders need not manage or even attempt to be good at management. 

Albert Mohler in "Conviction to Lead" sums up this danger very well in his chapter "Leaders Are Managers": 

Leaders are stewards of vision, conviction, beliefs & strategic decisions. Who cares about the color of the loading dock? Vision requires leadership. The delivery schedule can be left to managers, who need not worry about organizational strategy. 

But there is a seductive danger here. While we can agree that many good managers are not really leaders in the visionary and strategic sense, leaders absolutely must manage. If not, the actual mechanics of the organization, its policies and procedures, will be in the hands of others. That means there will be a disconnect between conviction and operation. In other words, the organization will fail.

The attempt to separate leadership from management has led to a host of disasters, and it is easy to see why. Management delivers the goods and determines what the organization actually does. If this is severed from leadership, there is no assurance that the organization will deliver on its mission. As a matter of fact, there is every reason to believe that it will not. 

Leaders lead by definition, but they also lead by management. There are certain management tasks that cannot be delegated, or can only be delegated with adequate supervision and oversight. (underlined mine) 

A leader who expects managers to get the job done while he/she only gives visionary/strategic direction or get others to attempt something he/she is unwilling to do, will be ineffective as a leader.

So in conclusion, while a good manager is not expected to be a good leader, a good leader must be a good manager because an effective leader is responsible to develop both good leadership & management abilities.