Introduction of Coca Cola, 1886
A sickly-sweet syrup-like substance
Cocal cola typically contains 12-US-fluid-ounce (350 ml) can contains 38 grams (1.3 oz) of sugar (usually in the form of high fructose corn syrup)
At first was sold at about fivepence
The introductory price was near 5 pence
T'was a remedy, an all-round cure
Now across the globe is 'click, fizz, pour'
Drugstore soda fountains were popular in the United States at the time due to the belief that carbonated water was good for the health, and Pemberton's new drink was marketed and sold as a patent medicine, John Pemberton claiming it a cure for many diseases, including morphine addiction, indigestion, nerve disorders, headaches, and impotence.
People open a bottle 'click', it fizzes 'fizz' and then pours it in a glass to drink 'pour'.
By the name of an actor
Imogene Coca, although OP wants the British actor Steve Pemberton as the inventor of Coca-Cola was named Pemberton.
And the place of an activist
Mayilamma was an Indian social activist whose claim to fame was the campaign against Coca-Cola Company in Plachimada in Palakkad, Kerala.
Here the OP wanted Atlanta (where it was first created and sold) was home to Martin Luther King Jr.
Weird that there are more than 1 solution to the above 2 lines.
Became a worldwide phenomenon
For which you can open happiness
Everyone across the world enjoys a glass of 'open happiness' now. Open happiness being one of the marketing slogans of Coca-Cola.